High Efficiency Video Coding (HEVC) is a video coding standard that is a successor to the H.264/AVC (Advanced Video Coding) standard. One of its primary objectives is to provide better compression efficiency without detectable loss in visual quality. Similarly, HEVC uses spatial and temporal prediction, transform of the prediction residual, and entropy coding of the transform and prediction information. In accordance with HEVC, the basic processing unit is a coding tree unit (CTU) that is a generalization of the H.264/AVC concept of a macroblock (MB). Each CTU has an associated quadtree structure that specifies how the CTU is subdivided. This subdivision yields coding units (CUs). A CU uses either intra prediction or inter prediction, and is subdivided into prediction units (PUs). In addition, a nested quadtree, referred to as the residual quadtree (RQT), partitions one CU into transform units (TUs). Concerning the entropy encoding, HEVC employs context-adaptive binary arithmetic coding (CABAC). Transform coefficients of each 4×4 transform coefficient block within one TU is encoded using CABAC according to a selected scan pattern, such as a diagonal scan pattern, a horizontal scan pattern, or a vertical scan pattern. In general, a scan pattern is used to convert a two-dimensional (2D) transform coefficient block into a one-dimensional (1D) transform coefficient array, and also defines a processing order for encoding the transform coefficients.
At the encoder side, one or more syntax elements are encoded for each transform coefficient in a 4×4 transform coefficient block. The decoding process is an inverse of the encoding process. Hence, at the decoder side, one or more syntax elements are decoded for each transform coefficient in a 4×4 transform coefficient block. A conventional entropy decoder uses different scan procedures (SPs) to decide different syntax elements, respectively. Specifically, the conventional entropy decoder is configured to perform a scan procedure for each coefficient locations in one 4×4 transform coefficient block. Since one 4×4 transform coefficient block has 16 transform coefficients corresponding to 16 different coefficient locations, one scan procedure is repeated 16 times. That is, the repetition number of one scan procedure is 16. However, it is possible that the entropy encoder does not encode a specific syntax element for a specific transform coefficient, performing a specific scan procedure for one coefficient location corresponding to the specific transform coefficient is redundant, which may have an impact on the decoding efficiency. Thus, there is a need for an innovative entropy decoder design which is capable of reducing the repetition number of at least one scan procedure to improve the decoding efficiency.